Frequently, it is necessary to convert the rate of rotation of a shaft from one value to another and various mechanical transmissions have been developed to do so. Shafts, bearings, housings, skewed journals, cams, roller followers, radially acting springs and wobble members such as nutating idlers, swash plates and the like, are all fundamental machine elements that have been combined in many different transmission assemblies known in the prior art. For example, circular arrays of rollers are shown in U.S. Pat. Nos. 1,277,193; 1,444,717; 1,634,453; 4,620,456 and 4,620,457, to transmit torque between two engaging surfaces. The invention disclosed herein represents a new combination of fundamental machine elements operating under unique conditions to provide a transmission which secures advantages not attainable by the prior art.
Many prior art transmission systems employ gears and the like to effect a speed reduction. In two-element gearing, relatively few engaging teeth transmit torque loads through contacts that slide as they roll, thereby creating problems of inefficiency, cogging, backlash and noise. Attempts have been made in the prior art to employ three element engaging systems in order to distribute the torque load over a relatively larger number of contacts and reduce shaft velocity variations. The present invention also employs a three element engaging system, except that it is purposely designed to have rolling and not sliding contact, thus avoiding frictional forces that reduce the efficiency of prior art shaft speed reducers. Many of the prior art three element, speed-reducer devices employ rollers as a third element for transmitting torque between two engaging elements; however, such devices can not achieve high efficiencies because the design thereof includes engaging surfaces that force the rollers to slide as well as roll. While the present invention includes rollers in connection with the third element, it is purposely designed so that the various engaging elements thereof all include spherically-directed surfaces which do not force the rollers to slide as they roll, and thereby avoid frictional losses inherent in prior art shaft speed reducers.
In order for a three element engaging system to conjugate, each of the three engaging elements must have their axes offset from each other. One method of achieving an appropriate offset is to translate the axes a fixed distance from each other. U.S. Pat. No. 4,829,851 shows a three element shaft speed reducer in which axes of the elements are parallel, but not co-linear, thus producing epicyclic motions. However, epicyclic motions have undesirable translational accelerations and decelerations which cause vibrations and require counter-balancing masses to achieve dynamic balance. The only other method of offsetting the axes is to align them at an angle to each other. A number of prior art speed reducers employ angled elements described as experiencing the motion of nutation. See, for example, U.S. Pat. Nos. 3,590,659; 3,640,154; 4,620,456 and 4,620,457. As defined in the art, nutation is a result of changing angles of offset. However, it is undesirable to change the angle of the engaging element's offset axis, because at high-speed, dynamic imbalance will be induced by the angular accelerations and decelerations as the offset angle changes, leading to wear, noise and vibration problems.
In accord with the present invention, the axes of the engaging elements are offset relative to one another, except that they are purposely designed to maintain a constant offset angle and are not subject to vibration induced by the motion of nutation claimed in prior art shaft speed reducers.
U.S. Pat. No. 4,563,915 discloses a shaft speed reducer which includes what is described as a wobble member, which has a cam element associated therewith having axially directed cam lobes. The patent further discloses that the lobes engage rollers which are constrained to move along the surface of an imaginary sphere; however, since the crests of the axially directed cam lobes are at a slightly greater distance from the center of the imaginary sphere than the valleys, and since the rollers are at a fixed distance, the rollers will be forced to disengage and reengage with the cam surface causing wear, noise and undesirable reciprocating forces. Alternatively, if the rollers cannot disengage and reengage, then they must deviate from the spherical alignment causing them to bind against the roller support assembly.
U.S. Pat. Nos. 4,220,457 and 4,620,456 disclose three element speed reducer assemblies which include roller elements constrained to travel in an axially undulating, hypotrochoidal or hypocycloidal path and to engage correspondingly trochoidally or epicycloidally configured members. These elements produce sharp changes in velocity and displacement leading to noise, wear and inefficiencies.
Most of the prior art shaft speed reducer patents refer to the three element engaging system using gear terminology, but the invention disclosed herein is a gearless mechanical transmission, and only cam terminology can realistically describe the assembly of rollers within a cage as the follower element, and the engaging surfaces as spherically directed multi-lobed cams. Only with respect to the calculation of the shaft speed ratios are the number of lobes on a cam element somewhat analogous to the number of teeth on a gear. The engaging surfaces will henceforth be referred to as spherically directed multi-lobed cams.
The design of the spherically directed multi-lobed cams are extremely critical for efficient operation with zero backlash, and prior art shaft speed reducer patents fail to properly address how to design engaging surfaces to eliminate sliding friction. The invention disclosed herein illustrates and describes the critical pitch path of the spherically directed multi-lobed cams so that the follower is directed to roll without sliding.
The present invention employs engaging elements with undulating surfaces, but the surfaces are all designed to be spherically directed so that only angular displacement of the various elements is encountered thus avoiding translational vibrations as well as disengagement and reengagement problems associated with prior art shaft speed reducers. The transmission of the present invention eliminates sliding contact thereby minimizing frictional losses, binding and other sources of inefficiency, wear and noise. In general, prior art mechanical transmission devices describe the various shafts as being input or drive shafts and output or driven shafts. This is because their inefficiencies are so great they cannot be utilized in a reverse manner as a speed increaser by driving the output shaft at a high ratio. The mechanical losses in the present invention are so low that it can also function as a high ratio speed increaser; therefore, in the context of the present disclosure, the shafts of the transmission will be described in terms of high-speed shafts and low-speed shafts rather than as input shafts and output shafts.
It will thus be appreciated that the present invention provides a gearless mechanical transmission which can function to increase or decrease shaft speed rotation. The transmission is quiet, efficient and capable of transmitting large amounts of power. Furthermore, the transmission is economical to manufacture since it can be assembled within economically achievable tolerances, and toward that end the present invention further includes a tolerance compensator which will be described in greater detail hereinbelow.